(1) Field of the Invention
The present invention generally relates to control surfaces and more particularly to an articulated control surface system for creating a curved control surface.
(2) Description of the Prior Art
It is well known that control surface actuator noise, as well as flow separation induced noise created by current "rigid" control surfaces, are significant sources of unwanted noise on underwater and airborne vehicles. All present actuators exceed the maximum allowable noise levels established for the Far Term Torpedo. A variety of techniques have been used to reduce the amount of noise created by existing electro/mechanical actuators. In general, these efforts have concentrated on balancing and isolating the moving parts and gears as well as providing fixed hydro/aerodynamic fairings. Unfortunately, even in the best prior art designs, electro/mechanical actuator-driven control surfaces suffer from the following drawbacks:
(1) Electro/mechanical, hydraulic or pneumatic activation of the control surface results in gear and motor noise;
(2) Significant size and weight of actuators add to the overall size and weight of the vehicle, while the seals and bearings associated with these actuators impose concurrent depth limitations and require precise machining tolerances;
(3) Control surfaces have limited placement possibilities since the control surfaces must have through-hull drives for electro/mechanical or hydraulic actuator systems;
Further, the rigid nature of these control surfaces increases the size of the turbulent wake behind the control surface thereby generating significant flow noise levels. The flow noise is created by three mechanisms: (1) the turbulence directly radiating to the near and far field, (2) the induced noise due to the turbulent excitation of the control surface and the surrounding structure, and (3) interaction of the control surface wake with the propulor. The latter causes fin and structure re-radiation which is the dominant flow noise source. Dominant flow noise is affected by a multiplicity of structural factors including:
(4) Control hinge gap interrupting flow over the control surface and causing flow separation; and
(5) Increased shed vorticity and turbulent wake created by the rigid control surface, thereby inducing turbulent excitation of the rigid control surface.
(6) Flow disturbance in the wake of control surfaces where typical propulsors, i.e. propellers, are located thereby greatly contributing to turbulence ingestion noise.
Recently, a novel approach has been conceived at the Naval Underwater Systems Center in Rhode Island. This approach utilizes a unique metal alloy that changes shape in response to electrical impulses. Since these shape changes can be tailored to create a curved surface, no conventional electro/mechanical devices are required to provide the hydrodynamic turning forces necessary for vehicle control. This effect eliminates typical through-hull drives and allows essentially universal placement of the control surfaces since only electrical power wires are required to create movement. This flexibility in placement can be used to great advantage by locating control surfaces behind the propulsor for reduced noise and improved efficiency. The operation of the control surface using the cambering in this invention is essentially silent. The cambered shape of the control surface foil also improves the lift coefficient compared to an equivalent symmetrical foil at the same angle of attack. At an equivalent lift coefficient, the articulated, or cambered shape produces less flow separation and thus less flow noise than a conventional symmetrical foil.
The original device disclosed in a patent application, Ser. No. 07/591,532 and filing date Oct. 1, 1990, by M. Cincotta and R. Nadolink, described a shape memory alloy actuator embedded in an elastomer foil shape. This configuration has the disadvantages of low mechanical advantage and severe space limitation. The space limitations result in limiting the number and length of wires which can be used, thus limiting the maximum deflection angle which can be obtained on the control surface.
The operation of the alloy requires specific thermal cooling from the ambient environment and control of the cooling rate is the key to development of the concept. In small vehicles the space limitations of the invention proposed by Cincotta and Nadolink reduce the potential for establishment of natural convection cells in heat transfer fluid surrounding the wires, thus reducing the wire cooling rate.